1. Field of the Invention
The present invention relates to an intermediate transfer medium for use in electrophotographic image forming apparatus. In addition, the present invention also relates to an electrophotographic image forming apparatus using the intermediate transfer medium.
2. Discussion of the Related Art
Electrophotographic image forming apparatus include various seamless belt members such as fixing belts, image transfer belts and receiving material feeding belts. Among the electrophotographic image forming apparatus, full color image forming apparatus typically use an intermediate transfer belt, which receives four color toner images formed on one or more photoreceptors to form a combined color toner image thereon and then transfers the combined image to a receiving material to form a full color image thereon. Demand for such intermediate transfer belts rapidly increases as demand for full color image forming apparatus increases.
Thermoplastic resins, thermosetting resins, rubbers and elastomers are typically used for such intermediate transfer belts.
On the other hand, tandem image forming apparatus, in which four image forming units each including at least a photoreceptor, a charger, and a developing device are serially arranged so as to be opposed to an intermediate transfer belt, have been typically used for full color image forming apparatus to perform high speed color image formation. Such intermediate transfer belts are required to have the following properties:
(1) The belts are not deformed during an image forming operation to prevent occurrence of a color misalignment problem in that one or more of color toner images are not transferred to the predetermined positions of the intermediate transfer belt, namely, the intermediate transfer belt is required to have a high mechanical strength sufficient for enduring mechanical stresses over a long period of time; and(2) The belts have good flame resistance.
Therefore, polyimide resins, and polyamideimide resins are typically used for such intermediate transfer belts. Particularly, polyimide resins are preferably used because of having a good combination of creep resistance and durability.
Since polyimide resins have a high mechanical strength, intermediate transfer belts made of such polyimide resins typically have a high surface hardness. Therefore, a toner image present on a polyimide intermediate transfer belt receives a high pressure in a transfer process, thereby often causing an image omission problem in that the toner particles forming the toner image aggregate due to the high pressure applied thereto, and part of the toner image is not transferred. In addition, polyimide intermediate transfer belts have a poor contact property (i.e., poor cushionability). Therefore, when a toner image is transferred from a photoreceptor to a polyimide intermediate transfer belt or from the intermediate transfer belt to a receiving material, the intermediate transfer belt tends to be unevenly contacted with the photoreceptor or the receiving material at the image transfer positions. In this case, an uneven image transfer problem is easily caused.
Recently, various image receiving materials are used for electrophotographic image forming apparatus. Specifically, not only plain papers having smooth surface, but also coated papers having high slip property and high smoothness, and papers having rough surface such as recycled papers, embossed papers, Japan papers and craft papers are used as receiving materials. In order to well transfer toner images onto such various papers, the intermediate transfer belt has to have a good cushionability. When an intermediate transfer belt having a poor cushionability is used, uneven density images and uneven color-tone images are produced.
In attempting to impart a good cushionability to an intermediate transfer belt, a technique in that a relatively flexible outermost layer is formed thereon is proposed.
Published unexamined Japanese patent application No. (hereinafter referred to as JP-A) 2001-100545 discloses an intermediate transfer belt in which an elastic layer is formed on a substrate, wherein the ratio of the thickness of the elastic layer and the thickness of the substrate is specified to prevent occurrence of the above-mentioned image omission problem. When a relatively thick elastic layer is formed on a substrate to impart good cushionability to the resultant belt, the substrate has to be also relatively thick, and therefore polyimide resins, which typically have a large elastic modulus, cannot be used as the substrate. Therefore, the intermediate transfer belt has poor creep resistance and durability.
JP-A 2001-125388 discloses a multilayer intermediate transfer belt, in which the volume resistivities of the substrate and the outermost layer are specified and the water absorption rate of the outermost layer is specified, to stabilize the volume resistivity of the belt even when environmental conditions change. However, the materials used for the intermediate transfer belt are popular materials and are not special materials. In addition, other properties of the belt are not explained therein.
JP-A 2004-354716 discloses an intermediate transfer belt in which a binder layer having a smaller elastic modulus than that of the substrate is formed on the substrate, and in addition a particulate material is adhered to the surface of the binder layer to impart a good combination of transferability and durability to the belt without deteriorating the flexibility of the belt. Although the particle diameter of the filler is specified, it is difficult to control the state of the thus adhered filler. In addition, since such a filler is easily abraded, the intermediate transfer belt has poor durability.
JP-A 2005-266793 discloses an intermediate transfer belt in which a thermosetting resin layer having a melting pint of not higher than 300° C. is formed on a thermoplastic resin substrate while hardened using a hardener. It is described therein that by forming an intermediate layer having elasticity is formed, the resultant intermediate transfer belt can be contacted with a photoreceptor or a receiving material while having a wide contact area, and thereby good image transfer efficiency can be imparted to the intermediate transfer belt. However, the thermosetting resin layer is formed to improve the scratch resistance of the belt, resulting in prevention of deterioration of glossiness of images. Therefore, the invention is different from the present invention. In addition, since a thermoplastic resin is used for the substrate, the substrate tends to easily deform or change the resistivity thereof when the thermosetting resin layer is hardened. Thus, the intermediate transfer belt has poor production stability.
JP-A 2006-285048 discloses an intermediate transfer belt in which an outermost layer including a hardened material of a liquid silicone rubber and carbon black is formed on a substrate made of a material such as polyimides and polyamideimides. It is described therein that the outermost layer has a good combination of releasability, elasticity and surface property. However, since silicone rubbers have poor adhesion to polyimides, an adhesive layer has to be formed between the outermost layer and the substrate. In addition, since carbon black is not well dispersed in silicone rubbers, the resultant intermediate transfer belt has uneven resistivity, resulting in occurrence of uneven image transfer. Therefore, the intermediate transfer belt has poor production stability.
Because of these reasons, a need exists for an intermediate transfer belt which has a good combination of creep resistance, dimension stability and durability and which can well transfer toner images onto various receiving materials having different surface properties, resulting in formation of high quality images without omissions and unevenness such as image density unevenness and color-tone unevenness.